This invention relates to methods for identifying anti-cancer therapeutics.
The induction of many types of cancer is thought to be ultimately caused by the products of activated cellular oncogenes (Bishop, J. M., Science 235:305-310, 1987; Barbacid, M., Ann. Rev. Biochem. 56:779-827, 1987; Cole, M. D., Ann. Rev. Genet. 20:361-384, 1986; and Weinberg, R. A., Science 230:770-776, 1985). Such oncogenes express oncoproteins that reside in the cell, often localized to specific cellular compartments such as the nucleus, cytoplasm or cell membrane.
In particular examples, the vital and cellular fos and myc oncogenes encode the Fos and Myc proteins, respectively. Expression of a large amount of either Fos or Myc in a variety of cell types allows the cells to grow indefinitely in cell culture (reviewed in Bishop, J. M. Cell 42:23-38, 1985; and Weinberg, R. A., Science 230:770-776, 1985). Overexpression of either Fos or Myc in normal rat fibroblasts, together with expression of an activated ras oncogene product, transforms the fibroblasts and endows them with the ability to form tumors in living animals (Land, H. et al., Nature 304:596-601, 1983; Ruley, H. E., Nature 304:602-606, 1983).
Fos and Myc are examples of oncoproteins that are phosphorylated, localized to the cell nucleus, and possess an ability to alter transcription (Donner, P. et al., Nature 296:262-266, 1982; Watt, R. A. et al., Mol. Cell Biol. 5:448-456, 1985; Renz M. et al., Nucl. Acids Res. 5:277-292, 1987). Recent studies suggest that oncoproteins such as Fos and Myc alter gene expression and immortalize cells by regulating the promoter activity of specific target genes and thus activating or repressing transcription of those target genes (see, for example, Varmus, H. E. Science 238:1337-1339, 1987; Kingston, R. E. et al., Cell 41:3-5, 1985; Bishop, J. M., Cell 42:23-38, 1985; Weinberg, R. A., Science 230:770-776, 1985).
Other oncoproteins are also thought to be involved in gene regulation. For example, the nuclear-localized oncogene product of v-jun (viral jun) binds to specific sites on DNA (Struhl, K., Cell 50:841-846, 1987) and is homologous to the c-jun (cellular jun) product, which also binds specific sites on DNA. The jun gene product appears to be identical to the previously characterized transcription factor AP-1 (Bohmann, D. et al. Science 238:1386-1392, 1987).
It is desirable to identify compounds that inhibit oncoprotein activity. By inhibiting oncoprotein activity, inhibition and/or control of oncoprotein-induced cell growth may be achieved. Especially, it is desirable to identify inhibitors of oncoproteins that do not alter the activity of the normal cellular equivalents of the oncoproteins. Administration of such inhibitors would provide therapeutic benefits in the treatment of diseases in which expression and activity of the oncoprotein is a factor in promoting cell growth or in maintaining the cell in a transformed state.
However, to date, very few oncoprotein inhibitors have been identified. The identification of such inhibitors has suffered for lack of a simple, inexpensive and reliable screening assay that could rapidly identify potential inhibitors and active derivatives thereof. The present invention provides such an assay. Using the instant assay, inhibitors of oncoprotein activity can be identified and, if desired, those inhibitors further screened to eliminate ones which generally inhibit normal cellular transcription or which inhibit activity of the normal cellular oncoprotein homologs.